In the quantitative analysis of materials by NIR spectroanalysis, a sample material is irradiated with infrared light and the intensity of infrared light reflected by or transmitted through the sample is measured at narrow band wavelengths distributed through the near infrared spectrum. When the sample is a solid dry material, the percentages of the constituents of the sample can be accurately measured by this technique; however, when the sample is a liquid sample, and, in particular, an aqueous liquid sample, the accurate measurement of the constituents of the sample becomes much more difficult because the water in the sample is perhaps the most strongly absorbing and temperature dependent material in the near infrared spectrum. Because of these characteristics, the presence of water in the sample causes background noise, reduces the accuracy of measurements on the sample and significantly reduces the ability of the analysis to detect constituents that are present in small percentages.
To overcome the problem presented by water in aqueous samples, it has been proposed to deposit the liquid sample on a porous base, such as a piece of filter paper and then dry the sample with hot air. The spectroanalysis is then performed on the dried concentrate. However, because of the problems of obtaining uniform drying, it has proved difficult to obtain accurate measurements by this technique.
Professor Marc Meurens has developed a technique in which the filter paper is supported on a wire screen supported in turn on top of a hollow cylindrical column. Heated air is then directed toward the top of the filter containing the deposited sample in a manner to allow the heated air to pass around the filter as well as through the filter so as to dry the sample. The technique achieves relatively uniform drying of the sample deposited in the filter and achieves measurements of the constituents of the original liquid sample which are accurate within about one percent. While this accuracy is satisfactory for many applications, a higher degree of accuracy is needed for many measurements.